1) Field of the Invention
The present invention relates to adhesive tapes for electronic parts to be used as adhesive tapes for fixing the leadframe, TAB tapes or for adhering between parts around leadframes making up a semiconductor device, e.g., lead pins, semiconductor chip mounted substrates, heat spreader, semiconductors themselves.
2) Description of the Related Art
Conventionally, adhesive tapes for fixing leadframes, TAB tapes, etc., are used as adhesive tapes for use in the interior of resin-molded type semiconductor devices. For example, the adhesive tapes for fixing leadframes have been used to fix lead pins of a leadframe in order to enhance the efficiency for producing the leadframe itself and the whole semiconductor assembly stages. In general, a leadframe manufacturer tapes the leadframe, and brings it to a semiconductor manufacturer, at which a semiconductor chip is mounted thereon followed by molding the leadframe with a resin. For this reason, it has been required for the adhesive tapes for fixing the leadframe not only to possess general reliability in a level required for semiconductor and enough processability at the time of taping, but also to possess sufficient adhesive strength at room temperature immediately after taping and heat resistance enough for withstanding thermal process at the stages of assembling semiconductor devices.
Conventionally, the adhesive tapes for use in such an application include adhesive tapes applying on a support film of a polyimide film, etc., an adhesive comprising polyacrylonitrile, polyacrylate or a synthetic rubber resin such as acrylonitrile-butadiene copolymer solely, or modified with any other resin or blended with any other resin to come into a B-stage. Both-sided adhesive tapes using thermoplastic polyimide resin having high reliability and high heat resistance have been utilized recently.
In recent years, resin-molded type semiconductor devices (semiconductor packages) as shown in FIG. 3 have been developed and produced. In FIG. 3, the device has a construction in which lead pins 3 and a metal plane 2 are connected by means of an adhesive layer 6, a semiconductor chip 1 is mounted on the metal plane 2, and together with bonding wires 4 between the semiconductor chip 1 and the lead pins 3, they are molded with a resin 5. In general, unilayer adhesive film or both-sided adhesive tape is used as the adhesive layer 6.
In the adhesive layer in the resin-molded type semiconductor devices shown in FIG. 3, the use of an adhesive tape to which a conventional adhesive is applied has the problems that the generated gas stains the lead to cause deterioration of the adhesive strength or causes generation of package crack, because of insufficient heat resistance. It is therefore required to develop adhesives for electronic parts having sufficient heat resistance and reliability and adhesive tapes for electronic parts using them.
The present inventor has solved before the above problems by inventing adhesive tapes using an adhesive containing a polyimide composed of repeating units represented by the below-described formulas (1a) and (2). (Japanese Patent Application Laid-open Nos. 325533/1996 and 67559/1997).
These adhesive tapes however have other various problems. For example, there is a problem of easily causing interfacial separation of the adhesive layer from the heat resistant film in the case that the adhesive layer is formed on a heat resistant film. In particular, the interfacial separation at high humidity and high temperature becomes a serious problems because of causing remarkable deterioration of reliability of semiconductor packages. In the case of an adhesive tape consisting only of an adhesive layer, there is the problem that insulation becomes difficult to assure when the tape is bonded under pressure with heat, because the leadframe is embedded in and pierces the adhesive layer.
In the prior semiconductor devices, the leadframe and the plane previously prepared are bonded under pressure with heat by means of a both-sided adhesive tape which was cut by means of a dies. It is therefore necessary to change the dies for cutting the adhesive tape according to the shapes of the leadframe and the plane, by which the process requires many production stages to increase the price of the semiconductor device.
Japanese Patent Application Laid-open No. 291236/1994 discloses a method of bonding the heat radiator to the lead frame in the semi-conductor device with an adhesive tape comprising polyimide adhesive layers having each a different glass transition temperature (Tg). However, there is no detailed description concerning component of the adhesive to be used. Since adhesives have each a different molecular structure, the disclosed semiconductor device has the problem that interfacial separation between or on the adhesive layers is easily caused to result in insufficient adhesion between the adhesive layers and insufficient electric reliability.
It is further proposed to laminate adhesive layers on a metal foil or plate, whereby the insulation is secured from the adhesive layer near to the metal foil or plate and the adhesion is secured from the other adhesive layer. By the way, the adhesive layer for fixing the leadframe is required to be composed of a resin having a relatively low Tg in view of preventing oxidation of the leadframe, preventing appearance of strains and shortening an operation time for bonding. However, when the glass transition temperature of the resin is too low, there cause problems that the lead pin moves during assembling stages of the semiconductor and package cracks easily occur. On the other hand, when the resin is designed to have a higher glass transition temperature, it requires a higher temperature for bonding, or it is necessary a long period of operation time for bonding when the bonding is conducted at a low temperature for avoiding oxidation.
Japanese Patent Application Laid-open No. 140106/1998 discloses an adhesive tape for electronic parts comprising laminated adhesive layers of specified polyimides having each a different glass transition temperature. This adhesive tape had a problem that viscosity of the lower Tg adhesive layer became low to cause variation of thickness of the adhesive layer when the adhesive tape was bonded to the metal plate. On the other hand, when the metal plate was bonded to the higher Tg adhesive layer of the adhesive tape after the lower Tg adhesive layer was bonded to the leadframe, there caused a problem that the lead pins embedded into the lower Tg adhesive layer to cause shifting or lifting of the lead pins, shorting etc., because a higher temperature and a higher pressure were necessary for bonding under pressure.
The present invention has been made for the purpose of solving such problems in the prior arts. Namely, an object of the present invention is to provide an adhesive tape for electronic parts capable of adhesion at a relatively low temperature for a short time without generating gas and causing interfacial separation, with keeping electrical insulation and having sufficient reliability.
The first adhesive tape for electronic parts of the present invention is characterized by comprising a metal substrate and an adhesive layer A and an adhesive layer B laminated in order on at least a surface of said metal substrate, wherein said adhesive A comprises a polyimide consisting of 100-20% by mol of the repeating unit represented by the following formula (1a) and 0-80% by mol of the repeating unit represented by the following formula (1b), said adhesive B comprises a polyimide consisting of 100-40% by mol of the repeating unit represented by the following formula (1a) and 0-60% by mol of the repeating unit represented by the following formula (2), and the adhesive layer A and the adhesive layer B have each a different glass transition temperature: 
wherein Ar represents a divalent group selected from the following formulas containing aromatic rings: 
wherein R1, R2, R3 and R4 which may be identical or different represent each a hydrogen atom, an alkyl group having 1-4 carbon atoms or an alkoxy group having 1-4 carbon atoms, provided that R1, R2, R3 and R4 are not hydrogen atoms at the same time: 
wherein R is an alkylene group having 1 to 10 carbon atoms or xe2x80x94CH2OC6H4xe2x80x94, the methylene group of which attaches to Si, and n means an integer of 1 to 20.
The second adhesive tape for electronic parts of the present invention is characterized by comprising a metal substrate and an adhesive layer A, adhesive layer B and an adhesive layer C laminated in order on at least a surface of said metal substrate, wherein said adhesive A comprises a polyimide consisting of 100-20% by mol of the repeating unit represented by the above formula (1a) and 0-80% by mol of the repeating unit represented by the above formula (1b), said adhesive B comprises a polyimide consisting of 100-40% by mol of the repeating unit represented by the above formula (1a) and 0-60% by mol of the repeating unit represented by the above formula (2), said adhesive C comprises a polyimide consisting of 90-40% by mol of the repeating unit represented by the following formula (3a) and 10-60% by mol of the repeating unit represented by the following formula (3b), and the adhesive layer A, the adhesive layer B and the adhesive layer C have each a different transition temperature: 
wherein X is a tetravalent aromatic group selected from the group Consisting of 3,3xe2x80x2,4,4xe2x80x2-diphenyl sulfone structure, 3,3xe2x80x2,4,4xe2x80x2-biphenyl structure and 2,3xe2x80x23,4xe2x80x2-biphenyl structure, and Ar and R represent each the same meaning as described above.